Modern radio frequency (RF) spectrum analyzers include high speed analog-to-digital converters (ADCs) to measure signals that have high bandwidths. Some of these RF spectrum analyzers include a calibration source for calibrating responses of the ADCs and other subsystems of the analyzer over a measurement bandwidth. The calibration involves stimulating the subsystems with a calibration signal provided by the calibration source, and then synthesizing a calibration filter based on measured responses to the calibration signal. Applying the calibration filter to measurements that are subsequently acquired by the RF spectrum analyzer compensates for errors in the responses of the various subsystems to achieve high performance in the analyzer.
Achieving high performance in an RF spectrum analyzer also relies on applying the calibration signal to input circuits, down-converters, ADCs, and other sensitive subsystems of the analyzer that are referenced to a low-noise analog ground that is isolated from a noisier digital ground in the analyzer to which field programmable gate arrays (FPGAs) and other digital circuits in the RF spectrum analyzer are referenced.
Prior art RF spectrum analyzers typically include the calibration source in an FPGA that is physically separated from the ADC. This results in a corresponding physical separation between the analog ground of the ADC and the digital ground of the calibration source. Accurate calibration then relies on the calibration signal traversing the physical separation between the analog and digital grounds without adding noise to the analog ground and without degrading the quality of the calibration signal. For calibration signals that have high bandwidths, the physical separation between the analog and digital grounds that can be traversed without compromising the calibration of the analyzer is typically shorter than the distance between the calibration source and the ADC, or between the calibration source and the other subsystems to which the calibration signal is applied.
To traverse this physical separation between the grounds, prior art RF spectrum analyzers include circuitry to receive and re-transmit the calibration signal on the analog ground at positions close to where the analog and digital grounds are physically separated. The RF spectrum analyzers may also include common-mode chokes to maintain isolation between the analog and digital grounds. However, the common-mode chokes and additional circuitry associated with traversing between the analog and digital grounds typically increase the cost and complexity of the RF spectrum analyzer and can add constraints in the placement of components and subsystems within the analyzer.
The ADCs and other subsystems in RF spectrum analyzers and other high performance measurement instruments are typically more sensitive to noise on the analog ground at higher measurement bandwidths. As the trend in ADCs is toward increasing measurement bandwidths, there is a need for a calibration source that enables calibration signals to be applied to ADCs and other subsystems that are referenced to low-noise analog grounds, without adding noise to the analog grounds and without the added cost and complexity of circuitry associated with traversing physical separations between analog and digital grounds in the measurement instrument.